As vehicle emission standards increase in stringency, it has become necessary for engine control system designers to devise more sophisticated strategies for the handling of vapors generated by the evaporation of fuel contained within the tanks of the vehicle. This fuel vapor is usually stored in one or more canisters, which are regenerated by causing atmospheric air to flow through the canister with the resulting combined gas stream consisting of air and fuel vapor being inducted into the engine's air intake for combustion. If such regeneration of the canisters is not handled properly, the air/fuel ratio of the engine may be disturbed. This may create a problem because the tailpipe emissions of the engine or vehicle could very well increase if the resulting engine feedgas oxygen level falls outside an acceptable range.
Various schemes have been used for introducing fuel vapors into an engine air inlet in a controlled manner.
U.S. Pat. No. 3,610,221 to Stoltman discloses a system allowing vapors to be drawn into a carburetor through the carburetor's idle and off-idle ports.
U.S. Pat. No. 4,646,702 to Matsubara et al. discloses a system allowing fuel vapors to flow from a storage canister only when certain engine operating parameters are in a satisfactory range, but without sensing the mass flow of the vapor coming from the canister. Unfortunately, without knowing the mass flow of the fuel vapor, it is not possible to precisely control the resulting changes in air/fuel ratio caused by the vapor.
U.S. Pat. No. 3,690,307 to O'Neill discloses a system in which the amount of purge air flowing through the vapor collection device is governed by the magnitude of the air flowing through the engine itself; not attempt is made to assess the mass flow of the vapors coming from the storage device.
U.S. Pat. No. 4,763,634 to Morozumi discloses a system which adjusts the fuel/air ratio control algorithm during vapor collection canister purging. This system, too, suffers from the deficiency that the quality of the vapor is not assessed.
U.S. Pat. No. 4,700,750 to Cook discloses a hydrocarbon flow rate regulator which is responsive to the concentration of hydrocarbon vapor and controls the rate of purge air flow accordingly. The regulator of the '750 patent is not, however, responsive to the mass flow of fuel vapor, and thus does not permit a finer level of control of the air/fuel ratio as with the present invention.
A hydrocarbon vapor sensor according to the present invention utilizes a critical flow nozzle to precisely measure the mass flow through the sensor system.
U.S. Pat. No. 4,516,552 to Hofbauer et al. discloses an air flow measuring device for a fuel injection system which measures the volumetric flow but not the mass flow of air through the sensor.
U.S. Pat. No. 3,604,254 to Sabuda and U.S. Pat. No. 4,041,777 to Leunig et al. disclose critical flow devices for testing automotive carburetors. Critical flow nozzles have been used in certain exhaust gas recirculation control valves used by Ford Motor Company for many years. Such valves control the flow of recirculated exhaust gas without determining the actual mass flow through the system.
It is an object of the present invention to provide a hydrocarbon vapor sensor system for an internal combustion engine which has the capability of determining the mass flow of fuel vapor entering an air intake system from a storage canister, such that a precise level of air/fuel control will be enabled.
It has been determined that vehicles operating on fuels having a high percentage of methanol may present unique problems in terms of cold weather starting ability. A sensor system according to the present invention could be employed for the purpose of accurately metering collected fuel vapor for the purpose of starting an engine fueled on liquids such as M-85 comprising 85% methanol and 15% gasoline.
It is yet another advantage of the present invention that a system according to this invention will allow a vehicle to more precisely control air fuel ratio for the purpose of controlling tailpipe hydrocarbon and carbon monoxide emissions.
Other objects, features and advantages of the present invention will become apparent to the reader of this specification.